What is the law that states that the total momentum of an isolated system is constant?

The law stating that the total momentum of an isolated system remains constant is the law of conservation of momentum.

Momentum is the product of an object's mass and velocity, and it has both magnitude and direction. In an isolated system, no net external force acts on the objects, so momentum may move from one object to another but the system's total remains unchanged. In a collision, for example, one object can lose momentum while another gains an equal amount.

This principle follows from Newton's laws: internal forces occur in equal and opposite pairs, so their effects cancel when the entire system is considered. It applies to elastic and inelastic collisions, explosions, and separations. A common mix-up is conservation of energy, which concerns total energy rather than vector momentum. In modern physics, momentum conservation is also linked to the uniformity of space through Noether's theorem.